Count of pairs in an array whose sum is a perfect square | C # implementation of the approach ; Function to return an ArrayList containing all the perfect squares upto n ; while current perfect square is less than or equal to n ; Function to print the sum of maximum two elements from the array ; Function to return the count of numbers that when added with n give a perfect square ; temp > n is checked so that pairs ( x , y ) and ( y , x ) don 't get counted twice  ; Function to count the pairs whose sum is a perfect square ; Sum of the maximum two elements from the array ; List of perfect squares upto max ; Contains all the array elements ; Add count of the elements that when added with arr [ i ] give a perfect square ; Driver code
using System ; using System . Collections ; using System . Collections . Generic ; public class GFG { public static ArrayList getPerfectSquares ( int n ) { ArrayList perfectSquares = new ArrayList ( ) ; int current = 1 , i = 1 ; while ( current <= n ) { perfectSquares . Add ( current ) ; current = ( int ) Math . Pow ( ++ i , 2 ) ; } return perfectSquares ; } public static int maxPairSum ( int [ ] arr ) { int n = arr . Length ; int max , secondMax ; if ( arr [ 0 ] > arr [ 1 ] ) { max = arr [ 0 ] ; secondMax = arr [ 1 ] ; } else { max = arr [ 1 ] ; secondMax = arr [ 0 ] ; } for ( int i = 2 ; i < n ; i ++ ) { if ( arr [ i ] > max ) { secondMax = max ; max = arr [ i ] ; } else if ( arr [ i ] > secondMax ) { secondMax = arr [ i ] ; } } return ( max + secondMax ) ; } public static int countPairsWith ( int n , ArrayList perfectSquares , ArrayList nums ) { int count = 0 ; for ( int i = 0 ; i < perfectSquares . Count ; i ++ ) { int temp = ( int ) perfectSquares [ i ] - n ; if ( temp > n && nums . Contains ( temp ) ) count ++ ; } return count ; } public static int countPairs ( int [ ] arr ) { int i , n = arr . Length ; int max = maxPairSum ( arr ) ; ArrayList perfectSquares = getPerfectSquares ( max ) ; ArrayList nums = new ArrayList ( ) ; for ( i = 0 ; i < n ; i ++ ) nums . Add ( arr [ i ] ) ; int count = 0 ; for ( i = 0 ; i < n ; i ++ ) { count += countPairsWith ( arr [ i ] , perfectSquares , nums ) ; } return count ; } public static void Main ( ) { int [ ] arr = { 2 , 3 , 6 , 9 , 10 , 20 } ; Console . WriteLine ( countPairs ( arr ) ) ; } }